Danni...what does he weigh? If you were on the newborns the next size up is infant - are they the ones you have? A few GFs of mine found that they went straight to the crawler, the next size again.
If they are leaking out the back it is unlikely it is cos they are too big, more like too small. (absorbancy wise) unless you are not doing them tight enough and high enough (which I doubt)
How long does he wear them for? Is he a heavy wetter anyway? When is he leaking out - sitting in the car, overnight?